The first controllers
In the early 1920s airline traffic grew fast, and busy airfields faced a problem that had not existed before: several aircraft wanted to land and take off at the same time, and the pilots could not see each other. Croydon, the main airport of London, was the first to answer it. In 1920 a small tower with a radio transmitter began to give pilots information on the weather and on other traffic, and it is generally counted as the first air traffic control in the world.
Controllers at that time did not give orders. They passed on information, and the pilot decided. The radios were bulky, and many aircraft carried none at all. Signals to those who could not receive them were made with flags and signal lamps.
A man with flags
In the United States the first controller is often said to be Archie League, who from 1929 stood at the edge of the field at St. Louis and directed the aircraft with a pair of coloured flags: one for go ahead and one for stop. In 1930 Cleveland became the first airport with a tower using radio to talk to pilots in the US.
The profession developed within a few decades into a large and regulated organisation. The ideas that began with a man on the grass, that someone on the ground sees the whole airfield and can give an order, are still at its centre.